The present invention relates to a direction of arrival (herein after recited as DOA) estimation apparatus to estimate a direction of an arrival wave using an array antenna, and a variable directional signal receiving apparatus to vary directional characteristics of an antenna basing on a result of a direction estimation of the DOA estimation apparatus.
As one of a method of estimating a direction of an arriving wave with high accuracy using an array antenna composed of plurality of antenna elements, the MUSIC (Multiple Signal Classification) described in a paper xe2x80x9cMultiple emitter Location and Signal Parameter Estimationxe2x80x9d, IEEE Trans., AP-34, 3, pp.276-280 (1986) by R. O. Schmidt has been known. The method is an algorithm that estimates a direction of multiple waves in a same frequency band simultaneously with high accuracy. With reference to FIGS. 15 and 16, the DOA estimation apparatus using the conventional MUSIC method will now be described.
FIG. 15 is a block diagram showing a composition of a DOA estimation apparatus. A signal 102-1 . . . 102-M received by antenna elements 101-1 . . . 101-M, where M is the number of elements (M greater than 1), is converted at frequency converters 103-1 . . . 103-M connected to each antenna elements 101-1 . . . 101-M and the signals are phase detected and then are converted to complex base band signals 104-1 . . . 104-M composed of an orthogonal I, Q signals.
The each complex base band signals 104-1 . . . 104-M are converted from analog signals to complex digital signals 106-1 . . . 106-M by an analog/digital converters (A/D converters) 105-1 . . . 105-M.
A data transfer section 107 transfers data of each complex digital signals x1(k),x2(k), . . . ,xM(k) obtained from the complex digital signals 106-1 . . . 106-M at a sample time of kxcex94T (xcex94T is a sample interval) to a DOA estimation processing section 108 in a lump at a specified timing after storing specified sample time temporarily.
A DOA processing section 108 performs a computation from the output data of the data transfer section 107 basing on the MUSIC method and estimates a direction.
FIG. 16 is a block diagram of a conventional DOA processing section 108. A computation of covariance matrix means 109 makes a receiving vector X(k) expressed with formula 1 using the complex digital signals 106-1 . . . 106-M obtained from the data transfer section 107 and makes a covariance matrix R shown in formula 2 using the receiving vector X(k) of the sample time k=1 . . . N. The T indicates transposition, and the H exhibits complex conjugate transposition.
X(k)=[x1(k), x2(k), . . . ,xM(k)]Txe2x80x83xe2x80x83(formula 1)
                    R        =                              ∑                          k              =              1                        N                    ⁢                                    X              ⁡                              (                k                )                                      ⁢                                                            X                  ⁡                                      (                    k                    )                                                  H                            /              N                                                          (                  formula          ⁢                      xe2x80x83                    ⁢          2                )            
An eigenvalue computation means 110 gets eigenvalues xcex1 . . . xcexM by computing the eigenvalues of the covariance matrix R in descending order.
An eigenvector computation means 111 computes eigenvectors e1 . . . ,eM corresponding to the eigenvalues xcex1 . . . xcexM. In a case, a number of arriving waves is S, a DOA evaluation function computation means 112 uses (M . . . S) number of eigenvector matrix EN=[es+1, . . . ,eM] belonging to a subspace spanned by noise eigenvector (hereinafter noise subspace) relative to formula 3 and utilizes an orthogonal characteristics between a subspace spanned by signal eigenvector (hereinafter signal subspace) Es=[e1, . . . ,eM] spread by eigenvector e1, . . . ,eM and EN. Namely, the DOA estimation function F(xcex8) is defined as formula 4, that evaluates an orthogonal characteristics with EN at the time whenxcex8 is rotated from 0 to 360xc2x0, wherein a(xcex8) (steering vector) indicates complex response of an array antenna against the directionxcex8.
xcex1xe2x89xa7xcex2xe2x89xa7. . . xe2x89xa7xcexS greater than xcexS+1=xcexS+2=- - - =xcexM xe2x80x83xe2x80x83(Formula 3) 
F(xcex8)={aH(xcex8)ENENHa (xcex8)}xe2x88x921xe2x80x83xe2x80x83(formula 4)
In a case when xcex8 equals to an arriving angle, the DOA estimation function F(xcex8) idealistically becomes infinite value. Therefore, the direction of peak value from the result of the computation of F(xcex8) at the time of angle xcex8 is regarded as the DOA estimation of an arriving wave.
In general, since the number of arriving waves S is unknown, a judgement is made to determine the number of waves by distribution of eigenvalue or by setting a signal number judgement reference described in the paper M. Wax and T. Kailath, xe2x80x9cDetection of Signals by Information Theoretic Criteriaxe2x80x9d, IEEE Trans. On Acoustics, Speech and Signal Processing, Vol. ASSP33(2), pp.387-392, February (1985).
With such DOA estimation apparatus using above MUSIC method, that estimates the DOA accurately by signal processing using an algorithm of eigenvalue decomposition, an reciprocally correlative value of the receiving signals between the antenna elements is obtained with the covariance matrix. And, statistically, the accuracy becomes higher when the observation time is longer. As a result, the higher accuracy of the DOA estimation is obtained. In order to secure certain accuracy level of the DOA estimation, it has been a problem that an amount of memory storing the temporary data at the data transfer section becomes larger, since the amount of receiving data used for the DOA estimation becomes larger due to handling a large amount of receiving signal data from each antenna elements converted to digital signals by A/D converters.
And also, it is required to secure transferring speed at a data transfer section and computation ability of signal processing apparatus corresponding to the moving speed of the estimating object when estimating the DOA of an transmitter loaded on a moving object.
With such DOA estimation apparatus using MUSIC method, that estimates the DOA accurately by signal processing using an algorithm eigenvalue decomposition of array receiving signals of a covariance matrix, it has been a problem that arriving waves cannot be separated with high accuracy when reciprocal delay time between the multiple waves reflected from the earth or buildings is shorter than a symbol length since the rank of the covariance matrix is decreased due to the high correlation between the multiple waves.
In order to deal with this problem, the Spatial Smoothing method has been proposed in the paper of Pillai et al, xe2x80x9cForward/Backward Spatial Smoothing Techniques for Coherent Signal Identificationxe2x80x9d, IEEE Trans. On Acoustics, speech and signal processing, VOL.37, NO.1, 1989.
With the spatial smoothing method, however, it has been a problem that substantial effects can not be obtained since the freedom of array is decreased further than sub-arrayed in a case of a four-element circular array antenna or so due to taking the averaging processing of a covariance matrix from sub-arrayed linear array.
The purpose of the present invention is to simplify the composition of the DOA estimation apparatus, and to provide an apparatus that enables to decrease the transfer speed of a data transfer section to signal processing apparatus and at the same time to decrease the memory capacity for accumulation and to alleviate the computation capacity of signal processing apparatus by decreasing the amount of computation.
And also, in a case when application is not to estimate directions of all the multiple arriving waves but limited to estimate the direction of the maximum level among the multiple arriving waves, the present invention is to provide an DOA estimation apparatus to detect accurately an DOA of the maximum level among multiple waves, even when the number of antenna elements is small and the correlation between the arriving waves is high.
Furthermore, utilizing a result of estimation by a DOA estimation apparatus, the present invention is to provide a variable directional signal receiving apparatus that improves receiving quality by controlling directivity of an antenna.
The another purpose of the present invention is to provide the DOA estimation apparatus that can estimate a direction of a main wave accurately even with a simple array composition like four elements or so and under multiple wave environment where correlation waves exist.
Still another purpose of the present invention is to provide a variable directional signal receiving apparatus, a variable directional signal transmitting apparatus and a variable directional signal receiving and transmitting apparatus that improves receiving and transmitting quality by controlling directivity of an antenna, utilizing a result of estimation by the DOA estimation apparatus.
In order to deal with the aforesaid problems, the DOA estimation apparatus of the present invention comprises:
a plurality of A/D converters that converts a plurality of complex base band signals obtained from the receiving signals of plurality of antenna elements to each complex digital signals,
a plurality of low pass filter that output complex low pass signals with attenuated high frequency components of each complex digital signal,
a plurality of down samplers that output complex down sampled signal by down sampling the complex low pass signals multiplied by integral number of sampling interval of each A/D converters.
a data transfer section that accumulates each complex down sampled signals thus obtained from each antenna elements temporarily and then transfers the data.
The variable directional signal receiving apparatus of the present invention improves the receiving quality of receiving apparatus by controlling directivity of antenna based on the aforesaid output data from the data transfer section.
The composition of the DOA estimation apparatus of the present invention comprises:
an arrival wave number judgement section that judges the number of arriving wave taking into consideration the correlation among them,
a DOA estimation section that performs computation for the DOA estimation based on the conventional eigenvalue decomposition method such as MUSIC method using the number of arriving waves from the judgement section,
an arrival wave power estimation section that estimates the power of arriving wave,
a main wave direction estimation section that estimates the direction of maximum level of arriving wave by synthesizing the estimation results of the DOA estimation section and of arrival wave power estimation section.
Thus, the deterioration of the accuracy of the DOA estimation can be suppressed at minimum level by combining the power estimation even when there is a highly correlated wave among the arrival waves.